All 4 slots are for RAM(Memory)! So you shall find the other 2 are sticks of RAM as well. I dont know how much the motherboard can take in RAM, but i should think you can put the sticks of 128mb or 256mb in any slot.
Make sure you wear a wrist strap!! You pick you 100's of volts in static when you read the newspaper. And 1000's of volts in static when you walk across a certain type of floor(carpet i think). So one touch to the inside of your pc and you've got static built up and bang(or it wont go bang like but sh!), thats a bit of money down the drain.